P0171
System Too Lean (Bank 1)
medium
i Description
The engine is running too lean on Bank 1. The ECU has detected that it needs to add more fuel than normal to maintain the correct air/fuel ratio. This means too much air or too little fuel is reaching the combustion chamber.
? Common Causes
- Vacuum leak (intake manifold gasket, brake booster hose, PCV valve)
- Dirty or faulty MAF sensor
- Low fuel pressure (weak pump, clogged filter)
- Exhaust leak before O2 sensor
- Faulty O2 sensor reading lean
! Symptoms
- Check engine light
- Rough idle
- Hesitation on acceleration
- Poor fuel economy
- Engine may stall at idle
- Possible lean misfire
# Diagnostic Steps
- Check long-term fuel trim (LTFT > +15% indicates lean)
- Smoke-test intake system for vacuum leaks
- Clean or test MAF sensor
- Test fuel pressure at rail
- Check for exhaust leaks upstream of O2 sensor
- Inspect PCV valve and hoses
⚙ Typical Fix
Find and repair the vacuum leak, clean or replace the MAF sensor, or address low fuel pressure.
Parts Needed
Varies: vacuum hoses, MAF sensor, fuel filter, intake gaskets
Estimated Cost
$100-$500
↔ Related Codes
Q Frequently Asked Questions
You can drive, but the lean condition may cause misfires and catalytic converter damage over time. A minor lean condition is OK for a short while; a severe one can overheat engine components.
If it is a vacuum leak, you might find it by spraying carb cleaner around the intake while idling — a change in RPM reveals the leak. MAF cleaning is also DIY-friendly. Fuel pressure testing requires a gauge.
Vacuum hose repair: $20-$50. Intake gasket: $150-$400. MAF sensor: $80-$300. Fuel pump: $300-$600.
☞ Learn More
- What is OBD-II? — Complete guide to your car's diagnostic system
- How to Read DTC Codes — Step-by-step with a Bluetooth scanner
- OBD-II Scanner Buying Guide — Find the best scanner for your car